Mon Nov 3. Steve Warren, regional climates water cycle ocean and solid earth circulations Friday: lab on reservoirs and cycles?

Transcription

1 Announcements: Midterm key on web (see Homeworks and Midterm button) Three chapters this week - see reading guide on HW#4 McCain-Leiberman, election day tomorrow Where we're going: Week 6: present climate Week 7: ancient climates, Earth history Week 8: recent climates, climate variability Weeks 9-11: future climates Monday and Tuesday: Earth as a heat engine three big pumps atmospheric circulations global distribution of temperature and precipitation Wed: Thurs: Steve Warren, regional climates water cycle ocean and solid earth circulations Friday: lab on reservoirs and cycles? Mon Nov 3

2 upcoming talks MONDAY 3 November 3:30 ATG 310c, Prof Sandy Tudhope, Edinburgh Univ ENSO: Evidence from living and fossil coral TUESDAY 4 November 12:30 ATG 310c, Weather discussion FRIDAY 7 November 3:30 15 OTB (Oceanography Teaching Bldg) Dr. Brent Helliker, Stanford "Terrestrial carbon cycle response to climate change"

3 Physical/Chemical tools Causes of air motion - bouyancy - pressure gradient force -friction -Coriolis force Air circulation - convergence/divergence (horiz.) - convection/subsidence (vertical) - conservation of matter Phase changes of water - evaporation/condensation - latent heat Ocean vs land heating rate - conduction - specific heat capacity - turbulent mixing - transmission of light Environmental phenomena Global and seasonal distribution of -temperature - precipitation Hadley cell -Trade Winds -ITCZ - subtropical deserts Land/Ocean circulations - monsoons - land and sea breezes Atmospheric water cycle

4 Layout of planet Earth name latitude range Tropics 0 to 30º Extratropics 30 to 90º Subtropics ~30º Midlatitudes 30-60º Polar Regions 60-90º portion of Earth surface 50% 50% 37% 13% Ocean 70% Land 30% Note: Most of Earth's surface is Tropics and/or Ocean. (These are the major components that a climate model needs to get right.)

5 Unequal distribution of solar energy with latitude: Fig 04_03 Recall: Flux is energy per unit surface area: W/m2

6 Net energy as a function of latitude: Fig 04_04

7 Satellite measurement of net energy: E IN -E OUT data source: Earth Radiation Budget Experiment

8 Earth as a Heat Engine energy OUT Eq. energy IN heat flow in atmos. & oceans Pole With no atmosphere or ocean currents, low latitudes would continue to warm and high latitudes would continue to cool. Atmosphere and ocean currents remove heat from Tropics and transport it to high latitudes. (Also from warm to cool regions on smaller scales - e.g. land/sea breezes.) These currents cannot flow in one direction only - air and water would "pile up". Result is "Circulation" (warm currents poleward, cool currents equatorward)

9 Heat transport from low to high latitude: Fig 05_16 Mechanisms: (i) general circulation of the atmosphere (actually, troposphere) (ii) surface ocean currents Note: These two are intimately connected.

10 Heat transport from low to high latitude: Fig 05_16 Questions: 1. Where is most heat transferred in the Tropics? 2. Where is most heat transferred in the Midlatitudes? 3. Why does the amount of heat transport decline so rapidly in the Polar Regions Tropics Midlatitudes Polar regions

11 Tropical Circulations - Move heat from source regions to sink regions - Have enormous consequences for regional/seasonal weather - Three big ones... Hadley circulation encompasses entire Tropics moves heat from low latitudes (near Equator) to higher latitudes (near 30º) Monsoons move heat between land and ocean regional/seasonal Walker circulation regional (but huge region) moves heat from warm Western Pacific to cooler Eastern Pacific strengthening and weaking of this is ENSO

12 Hadley Circulation - 1 buoyancy rising and falling (think of rubber ball in water vs rock in water) density mass per unit volume less dense fluid rises more dense fluid sinks gas law (see p. 60) warm air is less dense Eq.

13 Hadley Circulation - 2 pressure gradient force "gradient" refers to high and low pressure regions cause of horizontal air motions induces air to flow from high pressure to low pressure actual air motion is modified by: friction Earth rotation (Coriolis force) L Eq.

14 Hadley Circulation - 3 stratosphere very stable region vertical motion is inhibited acts as a lid stratosphere L Eq.

15 Hadley Circulation - 4 conservation of matter... >>>>> CIRCULATION The Hadley Circulation stratosphere 30ºS L Eq. 30ºN

16 Hadley Circulation - Fig 04_03 convection Horizontal motions convergence: divergence: coming together spreading apart Vertical motions convection: subsidence: rising air sinking air

17 IR satellite image ITCZ: Fig 04_07

18 Tues Nov 4 Upcoming talks: TUESDAY 4 November 12:30 ATG 310c, Weather discussion FRIDAY 7 November 3:30 15 OTB (Oceanography Teaching Bldg) Dr. Brent Helliker, Stanford "Terrestrial carbon cycle response to climate change" Yesterday: Hadley cell - major circulation and good example of a circulation Today: Tropical circulations wrap-up Land/ocean contrasts Water cycle (if time) Wed: Thurs: Steve Warren, regional climates in Tropics (in-class: questions) water cycle ocean and solid earth circulations Friday: lab on reservoirs and cycles?

19 Tropical circulations (focus on Hadley) Convection: precipitation and heat transfer Subsidence: clear skies, deserts Convergence: surface winds Seasonal variations due to orbital parameters Extra-tropical circulations Outline Land/ocean contrasts physical basis effects on wide range of time- and space-scales Water cycle main components and processes Box Models and Residence Time Goals understand distribution of climates around the world understand the challenges of "climate modeling"

20 George Hadley ( ) English physicist and meteorologist. Proposed theory planetary-scale circulation cells in a 1735 paper, "Concerning the Cause of the General Trade Winds" Speaking of news...

21 Hadley Circulation - convection Convection evaporation at surface phase change (liquid > gas) requires tremendous energy energy carried up as latent heat 30ºS specific heat of water: 2 J/g/ºC "It takes 2 Joules of energy to heat 1 gram of water by 1ºC." latent heat of water: 2500 J/g "It takes 2500 Joules of energy to evaporate 1 gram of water." stratosphere L Eq. 30ºN rising air expands and cools (see gas law, p. 60) this causes water to condense when RH=100% (saturation) clouds form latent heat is released, causing the cloudy air to warm becomes less dense and more buoyant rises even faster >> towering cumulonimbus (thunderstorms)

22 Convection and the energy budget Huge amounts of energy (and moisture) are transported into the atmosphere by convection.

23 Hadley Circulation - subsidence Subsidence sinking air compresses and warms this suppresses cloud formation absence of rain - deserts stratosphere 30ºS L Eq. 30ºN

24 Hadley Circulation - convergence Low-level Convergence air forced upwards: "ITCZ" (Inter-Tropical Convergence Zone) horizontal motion modified by -friction - Earth's rotation (Coriolis Force) Coriolis: wind (or ocean current) veers right in N Hemi. 30ºS stratosphere L Eq. 30ºN non-rotating planet Earth

25 Orbital parameters and seasons: Fig 4-15 obliquity: tilt of Earth's axis with respect to orbital plane

26 Hadley - seasonal movement: Fig 04_16 June/July: ITCZ north, wet season in N Hemi Tropics Dec/Jan: ITCZ south, wet season in S Hemi Tropics

27 Seasonal surface pressure Fig 4-18 Dec/Jan: ITCZ south, wet season in S Hemi Tropics June/July: ITCZ north, wet season in N Hemi Tropics

28 Seasonal Precipitation Dec/Jan: ITCZ south, wet season in S Hemi Tropics June/July: ITCZ north, wet season in N Hemi Tropics

29 Deserts: Fig 04_24 30N 30S Causes: - descending arms of the Hadley cell (roughly +/- 30º) - continental interiors (far from water source) - leeward (downwind) slopes of mountains - west coasts with cold ocean (fog and low cloud but no rain)

30 Tropics vs extratopics, vertical profile, Fig 04_06 Tropics: - surface heating drives convection Extra-Tropics: - colliding air masses drives convection - warm air rises over cold (density effect) - massive heat transport in atmosphere

31 Extra-Tropical Cyclones (frontal systems)

32 Land/ocean contrasts Ocean surfaces change temperature far more slowly than land surfaces. WHY??? Four reasons: thermal conductivity is higher for water heat is transferred downward (away from surface) more efficiently in water specific heat capacity is higher for water takes more heat to change temperature of water transmission of solar energy to greater depth in water energy penetrates many meters in water vs a few mm turbulent transfer of heat (absent for land) surface water is mixed downward, transferring heat away from the surface Which is most important??? Turbulent mixing. To warm (or cool) the ocean surface, you have to warm a layer ~100 m thick vs only a few mm for the land.

33 Diurnal effect: sea/land breeze (Fig 04-17) L L land (warm) sea land (cold) sea

34 Seasonal effect: "Find the continents game" Contour lines show seasonal temperature range (like Fig 4-1c of text) compare: Tropics vs Extratropics Land vs Ocean

35 Decade-to- Century-Scale Effect lagged response to climate forcing arises mainly due to time it takes to heat the surface ocean (combined with feedbacks)

36 Land/Ocean Contrasts - Summary Illustration of a "coupled system": The oceans have an enormously stabilizing effect on atmospheric temperature. This arises primarily because of turbulent mixing of the ocean surface layer. This turbulent mixing (as we will see in Chap 6) is caused by atmospheric winds.

37 Thurs Nov 6 Upcoming talks: FRIDAY 7 November 3:30 15 OTB (Oceanography Teaching Bldg) Dr. Brent Helliker, Stanford "Terrestrial carbon cycle response to climate change" Today: Some clarifications Water cycle / Residence Time Surface Ocean currents Deep ocean circulation ("Thermo-Haline Circulation" or THC) Solid Earth circulation (plate tectonics, Wilson cycle) Friday: lab demos on reservoirs, cycles, Coriolis, clouds, etc >>> LOCATION: ATG 116 (Machine Shop) <<<

38 Land/ocean contrasts Four reasons why ocean changes temperature more slowly than land thermal conductivity is higher for water heat is transferred downward (away from surface) more efficiently in water specific heat capacity is higher for water takes more heat to change temperature of water transmission of solar energy to greater depth in water energy penetrates many meters in water vs a few mm turbulent transfer of heat (absent for land) surface water is mixed downward, transferring heat away from the surface Most important? Turbulent mixing. To warm (or cool) the ocean surface, you have to warm a layer ~100 m thick vs only a few mm for the land.

39 Water Cycle Picture: Fig 4-22a advection: Horizontal transport by atmospheric winds. Can apply to a substance, energy, or a property like temperature.

40 "Box Model" of the Water Cycle: Fig 4-22b reservoir: Specified location in which some material substance is found. (One "box" in a "box model" representation of a system.) burden: The total amount of material within a given reservoir. source (sink): The rate at which material is added to (removed from) a reservoir. residence time: The average amount of time material spends within a reservoir.

41 Residence Time (or Lifetime) Residence Time: The average amount of time material spends within a reservoir. Standard method of calculation: Residence Time = Burden Sink This simple formula provides a powerful means of evaluating the time scale for change in a system. We will use it extensively in discussing the carbon cycle and global warming. On HW #4, you are asked to calculate the residence time of water in the atmosphere.

42 What is the Residence Time of water in the ocean? Residence Time = Burden / Sink Burden = 1350e15 m 3 Sink (due to evaporation) = 361e12 m 3 /yr 1350e15 m RT = 3 = 361e12 m 3 /yr = 3.7e3 yr = 3700 yr 1350e3 yr 361

43 Surface Ocean Currents: Fig 5-2 wind-driven circulating "gyres" in each ocean basin

44 Surface Ocean Currents: Fig 5-3 warm and cold currents along continents influence regional climates - e.g. cold currents on Western margins enhance desert transport heat poleward... HOW?

45 Surface Ocean Currents: Fig 5-3 driven by wind, specifically, by friction of air moving over water causes turbulent, vertical mixing of upper ocean well-mixed surface layer is ~100 m thick. HOW MANY ATMOSPHERES?

46 Net Poleward Heat Transport: Fig 5-16 ocean currents transport a large fraction of total heat, especially in the Tropics

47 Thermo-haline circulation (THC): Fig 05_12 causes Deep Ocean mixing very long timescale: ~1000 years driven by density variations

48 Thermo-haline circulation (THC) Ocean water density - two controlling factors: temperature (cold water is more dense) salinity (more saline water is more dense) warm, "fresh" water stays on the surface cold, saline water sinks

49 Thermo-haline circulation (THC): Fig 05_12 Northern Atlantic Ocean is a key region where cold, saline water develops this may be what initiates the THC possible "trigger" point for global climate

50 Continental drift: Fig 6-1

51 Plate tectonics: Fig 6-21 another "pump" driven by circulations in the upper mantle; ultimately, by radioactive decay, releasing heat within the Earth's interior recycles key substances like mineralized carbon

52 Wilson Cycle: Fig 6-27 continents group together then spread apart timescale is ~500 million years major climatic consequences (e.g. linked to "Snowball Earth" and mass extinction events)

53 Summary Three BIG Pumps Atmosphere/Surface Ocean - distributes heat poleward - cause of regional and seasonal climates - mixing timescale is ~1 week THC - mixes deep ocean - timescale of mixing is about 1000 years - may shut on and off as conditions change in N. Atlantic - possible "trigger" for global climate Wilson Cycle - continents group and then spread - timescale is ~500 million years - major climatic effects - recycles key material from rocks back to the atmosphere